This section illustrates useful background information without admission of any technique described herein representative of the state of the art.
Cellular networks or more accurately cellular telecommunications networks are ubiquitous in modern societies. To enable a cellular terminal to start communications over a cellular network, the cellular terminals need to attach or register to the network in a network registration process. In the network registration process, a cellular terminal exchanges signals to authenticate itself or more accurately its subscription, typically using a USIM application on a UICC. In the network registration process, the cellular terminal obtains from the network and the SIM access information such as a session key with which the cellular terminal can subsequently communicate in the cellular network. The access information typically changes to prevent re-use of the access information by a possible illegal interceptor.
Encryption is a basic tool that is employed also in other types of digital cellular systems. Already GSM used encryption to enable mitigating illegal interception. The development of computer technology has subsequently made old encryption techniques more vulnerable, but also helped to enhance the security techniques used in cellular systems. For instance, wide-band CDMA (W-CDMA) was designed for stronger security by enabling also the network to authenticate itself to the cellular terminals. In the W-CDMA, the subscriber identity is provided by a Universal Integrated Circuit Card (UICC) that runs a Universal Subscriber Identity Module (USIM). The USIM produces e.g. a session key based on a shared secret stored on the UICC, challenge and replay attack prevention codes received from the network and cryptographic algorithm that is enhanced over the one used in GSM. Also the authentication signaling is enhanced in the W-CDMA over GSM e.g. for protection against some man-in-the-middle attacks.
In parallel with the development of security methods for securing the communications in the cellular systems, there are also growing needs for developing the structure of cellular terminals. At present, most cellular terminals contain an identity module slot such as a SIM slot in which a user can place and replace an identity module card such as the UICC. There is also development towards identity modules that are not physically replaceable so as to enable over-the-air change of subscription and/or to prevent theft of the identity module from a cellular terminal. Such software identity modules may be very useful e.g. for built-in vehicular communication systems so that their emergency reporting capabilities and possible burglar control systems could not be easily deactivated by removing an identity module card. While the non-removability of embedded UICCs brings its advantages with respect to theft protection or price, it has also its challenges. One major challenge is the user of the machine that holds the identity module may wish to have another cellular operator. Today, the operator would issue a new card. This is not possible with embedded modules. Therefore, an operator is for the machine use cases not necessarily sure what kind of module will be connected to its network and what capabilities it has. This will lead to error cases that we don't have today and which need to be solved to avoid that the module cannot connect at all.
While necessary for security, the authentication signaling unfortunately delays completion of network registration procedures. Moreover, the inventors have now identified that in some particular combinations of cellular terminal equipment, network configuration and encryption authentication protocols, a cellular terminal might engage into a perpetually failing loop so that its user could not establish telecommunications connectivity at all.